1. Field of the Invention
The present invention relates to a spin valve sensor with a low moment free layer and, more particularly, to a free layer which is composed of nickel iron molybdenum (NiFeMo) for high linear density reading by a read head.
2. Description of the Related Art
The heart of a computer is a magnetic disk drive which includes a rotating magnetic disk, a slider that has read and write heads, a suspension arm above the rotating disk and an actuator arm that swings the suspension arm to place the read and write heads over selected circular tracks on the rotating disk. The suspension arm biases the slider into contact with the surface of the disk when the disk is not rotating but, when the disk rotates, air is swirled by the rotating disk adjacent an air bearing surface (ABS) of the slider causing the slider to ride on an air bearing a slight distance from the surface of the rotating disk. When the slider rides on the air bearing the write and read heads are employed for writing magnetic impressions to and reading magnetic signal fields from the rotating disk. The read and write heads are connected to processing circuitry that operates according to a computer program to implement the writing and reading functions.
An exemplary high performance read head employs a spin valve sensor for sensing the magnetic signal fields from the rotating magnetic disk. The sensor includes a nonmagnetic electrically conductive first spacer layer sandwiched between a ferromagnetic pinned layer structure and a ferromagnetic free layer structure. An antiferromagnetic pinning layer interfaces the pinned layer structure for pinning a magnetic moment of the pinned layer structure 90xc2x0 to an air bearing surface (ABS) wherein the ABS is an exposed surface of the sensor that faces the magnetic disk. First and second leads are connected to the spin valve sensor for conducting a sense current therethrough. A magnetic moment of the free layer structure is free to rotate upwardly and downwardly with respect to the ABS from a quiescent or bias point position in response to positive and negative magnetic field signals from a rotating magnetic disk. The quiescent position, which is parallel to the ABS, is the position of the magnetic moment of the free layer structure with the sense current conducted through the sensor in the absence of signal fields.
The thickness of the spacer layer is chosen so that shunting of the sense current and a magnetic coupling between the free and pinned layer structures are minimized. This thickness is typically less than the mean free path of electrons conducted through the sensor. With this arrangement, a portion of the conduction electrons are scattered at the interfaces of the spacer layer with the pinned and free layer structures. When the magnetic moments of the pinned and free layer structures are parallel with respect to one another scattering is minimal and when their magnetic moments are antiparallel scattering is maximized. Changes in scattering changes the resistance of the spin valve sensor as a function of cos xcex8, where xcex8 is the angle between the magnetic moments of the pinned and free layer structures. The sensitivity of the sensor is quantified as magnetoresistive coefficient dr/R where dr is the change in the resistance of the sensor as the magnetic moment of the free layer structure rotates from a position parallel with respect to the magnetic moment of the pinned layer structure to an antiparallel position with respect thereto and R is the resistance of the sensor when the magnetic moments are parallel.
In addition to the spin valve sensor the read head includes nonconductive nonmagnetic first and second read gap layers and ferromagnetic first and second shield layers. The spin valve sensor is located between the first and second read gap layers and the first and second read gap layers are located between the first and second shield layers. In the construction of the read head the first shield layer is formed first followed by formation of the first read gap layer, the spin valve sensor, the second read gap layer and the second shield layer. Spin valve sensors are classified as a top or a bottom spin valve sensor depending upon whether the pinning layer is located near the bottom of the sensor close to the first read gap layer or near the top of the sensor close to the second read gap layer. Spin valve sensors are further classified as simple pinned or antiparallel pinned depending upon whether the pinned layer structure is one or more ferromagnetic layers with a unidirectional magnetic moment or a pair of ferromagnetic layers that are separated by a coupling layer with magnetic moments of the ferromagnetic layers being antiparallel. Spin valve sensors are still further classified as single or dual wherein a single spin valve sensor employs only one pinned layer and a dual spin valve sensor employs two pinned layers with the free layer structure located therebetween.
The free layer structure typically employs a nickel iron layer which provides a desirable magnetic softness for the free layer. This means that the free layer has a low uniaxial anisotropy HK which promotes responsiveness of the free layer structure to signal fields from a rotating magnetic disk. When the free layer structure is highly responsive a small signal field will rotate the magnetic moment of the free layer structure which causes a change in the magnetoresistance of the spin valve sensor. It has been found that when the free layer structure also includes a cobalt iron or cobalt layer, sometimes referred to as a nanolayer, between the nickel iron layer and the spacer layer and interfacing the spacer layer that the magnetoresistance or magnetoresistive coefficient dr/R is improved. In order to obtain a desirable increase in the magnetoresistive coefficient dr/R, it has been further found that the thickness of the cobalt iron or cobalt layer should be on the order of 15 xc3x85. Unfortunately, any increase in the thickness of the cobalt iron layer reduces the softness of the free layer structure so that it is not as responsive to signal fields from the rotating magnetic disk.
The areal density of a read head is the product of linear bit density and track width density. Linear bit density is quantified as bits per inch (BPI) along the track of the disk and track width density is quantified as tracks per inch (TPI) along the radius of the disk. An increase in the areal density equates to increased storage capacity of a computer which has progressed from kilobytes to megabytes to gigabytes.
The linear bit density is increased by increasing the number of bits per inch along the tracks of the rotating magnetic disk. When this occurs, each bit from the disk produces a lower magnetic field which must be sensed by the spin valve sensor. In order to meet this objective the free layer of the spin valve sensor must be more sensitive to the reduced fields from the smaller bits along the track of the rotating magnetic disk. This means that the magnetic moment of the free layer must be reduced so that it is less stiff in its response to the field from the bits of the rotating disk. This is accomplished by reducing the thickness of the nickel iron layer in the free layer structure so that the magnetic moment of the free layer structure is less and therefore more responsive to lower field signals from the bits of the rotating magnetic disk. The thickness of the cobalt iron layer is preferably not reduced so that the aforementioned improved magnetoresistive coefficient dr/R is maintained. Unfortunately, several angstroms at the top and bottom surfaces of the free layer, as these surfaces interface the spacer and cap layers, intermix with these layers which makes the top and bottom portions of the free layer structure magnetically stiff. As the thickness of the free layer structure is reduced the ratio of these magnetically stiff portions to the central more magnetically soft portion of the free layer increases. Because of these magnetically stiff portions at the interfaces of the free layer structure, the magnetic softness of the free layer structure does not decrease in a desired manner as its thickness is reduced.
The present invention provides a free layer structure which does not have to be reduced in its thickness in order to be sensitive to high linear bit density recording. This is accomplished by making at least a portion of the free layer structure of nickel iron molybdenum (NiFeMo). In the preferred embodiment the composition is nickel iron molybdenum (Ni78Fe12Mo10). The moment of nickel iron molybdenum (Ni78Fe12Mo10) is two and one-half times less than nickel iron (Ni83Fe17). Accordingly, a nickel iron molybdenum (Ni78Fe12Mo10) free layer portion can be two and one-half times as thick as a nickel iron (Ni83Fe17) free layer portion with the same magnetic moment. This means that the ratio of the insensitive interface portions of the free layer structure is less and therefore does not impact the magnetic softness of the free layer structure nearly as much as simply reducing the thickness of a nickel iron (Ni83Fe17) free layer portion.
An object of the present invention is to provide at least a portion of a free layer of a spin valve sensor with a lower moment than nickel iron without reducing the thickness of the free layer structure.
Another object is to provide a method of making the aforementioned free layer structure.
Other objects and attendant advantages of the invention will be appreciated upon reading the following description taken together with the accompanying drawings.